Relaxation oscillator



Jan. 27, 1953 J. R. MOORE 2,627,031

RELAXATION OSCILLATOR Filed Nov. 27; 1946 I i- U U62 U84 a U84 I]? 3 H H H H 4 Bummer fimesR/Ifoore saw Patented Jan. 27, 1953 RELAXATION OSCILLATOR,

James R. Moore, Washington, D: 0., assignor to: Radio Corporation ofiimerica a corporationofl- Delaware Application November 27, 1946,,Serial No. 112,553

B CIaims'.

11 This inventionrelates to relaxationioscillators such as are used in sawtooth Wave generators and the like, and more particularly to relaxation oscillators wherein the frequency of" oscillation; is controlled by adjustment of a voltage applied thereto.

One of the objeQtsof the invention is to provide a voltage controlled relaxation oscillatorwhose frequency of operation is substantially independent of ordinary variationsin tube characteristics.

Another object is to provide an oscillator ofthe described type whose frequency may be varied throughout, a wider range without variation in output amplitude.

The invention will be described with reference to the accompanying drawing, wherein;

Figure 1 is a schematic circuit diagram of a relaxation oscillator embodying the invention,

Figure 2 is an oscillogram' showing the output wave providedby-the oscillator of Figure l, and

Figures 3 and. 4 are further oscillograms of voltages appearing at certain points in the circuit ofFigure 1 during its operation.

Ordinarily, a relaxation oscillator includes a storage device, such as a capacitor, which is charged at; a relatively slow rate during the major portion of each cycle, and discharges as nearly instantaneously as possible when the charge reaches a predetermined value. The

charge-discharge cycle recursat a frequencydetermined primarily by-the charging-rate. The present oscillator difiers from such prior art devices: in that the charging occurs substantially instantaneously, and the repetition frequency-is controlled by adjustment of the rate-of thedischarge.

Referring to; Figure 1, a capacitorl is shunted by an electron discharge tube: 3. A resistor 5 is included in the cathode circuit of the tube 3 to cause the tube 3 to act as a substantially-constant current" device, as described hereinafter.

The capacitor I is connected -throughan electron dischargetube I and a resistor 9 to'a D;-C; supplysource havinga potential of, for example, 600 volts. The control grid of-the tube I isconnected to the anode of a tube II, which may be of the pentode type. The tube II is provided with a load resistor I3, and with screen grid and cathode bias-resistors I5 and I1 respectively. The anode supply for the tube II is lower than that of the tube 1-, being for example 400 volts; The control grid of the tube II is coupled through a capacitor I 9- to the anode of the-tubal.

The upper terminal of the storage capacitor I;- isconnected to the control grid of'atube 2 I; The tube 2| is arranged as a cathode follower with its anode connected directly to the positiveterminal of 'the D.-C".' supply (600V01ts in thepresenti example) andwith, a load resistor 23 in itscathode circuit. A further tube 25; is provided with its cathode connected to thatof' the tube H and; its anode connected tothe anode of the tube-la The control grid of'the tube 25"is;b1assedjfrom, a tap on a voltage divider 2lg connected across the -D.-C; supply.-

The voltage at the control gridrofthetube 3 determines the frequency; of operation" of the oscillator as willbe explained hereinafter. In: the present illustration-, this voltage is" proyidedi by an adjustable voltage divider 29' connected:

across the 400 voltsupply:

An additional tube 3 I- is included; Withdts-"con' trol grid and cathode tiedtc the corresponding elements of the tube 1'; and its: anode connected directly to the positive high voltage supply ter minal,

The adj ustment and operation ofthe described circuit is as follows:

The voltage divider 27 is set to provide a bias eg at the control grid of the tube 253 Iletus assume that the capacitor I has already; been; charged'throug-h thetubes 1 and} I toavoltage greater than 63. As a resultof the cathodefol' lower action of the tube 2I; the voltage at the cathodeof the tube 25 is likewise greater thanea. hence the tube 25 is cut oil.-

The capacitor I discharges through the tube 3 at a rate depending uponthe valueof'ei. The grid-to-cathode voltage on the tube 3- is; the differencebetwcen c1 and thedrop in theresistori'; The tube 3 conducts enough: to maintain this dif ference small. Thus the current through the, resistor 5, and hence the discharge rate, issubstantially independent of'ordinary variations in the characteristics of the tube 3; and is practicallyconstant for aconstant value of e1; The voltage e2 across-the capacitor, I decreases, as" shown by the downwardly slopingportions-ofthe curve of Figure 2, until e2 is slightlyless than the bias e3. At this point, the cathode ofthe tube 25 is nolonger positive, with respect to the grid, and the tube 25 starts to conduct; The, current flows through the resistor 9, reducing the voltage a; at the anodes-oftubes I and 25;

See Figure 3. The drop in potential is applied anodetoincrease to the level of the400 volt sup ply, as illustrated in Figure 4. The voltage e5 is applied directly to the control grid of the tube 1, causing it to conduct because e5 is higher than the potential (substantially ea) at the cathode of the tube 1.

Since the grid-cathode circuit of the tube 3| is in parallel with that of the tube 1, the tube 3| will conduct also at this time. Current flows through the tubes 3| and I from the 600 volt source to the capacitor I, charging it rapidly until the voltage ez at the cathodes of the tubes 3| and I exceeds the voltage as at the control grids, i. e. about 400 volts. This is illustrated by the upwardly extending portions of the curve of Figure 2. During the charging period, the voltage drop in the resistor 9 holds the voltage at at a relatively low level. (Figure 3.)

When the tubes 3| and 1 stop conducting, the voltage 24 rises, causing the tube H to start conducting. This reduces the voltage e5, keepingthe tube 1 (and with it the tube 3|) cut off until e2 again decreases to the bias level as.

The charging portion of the cycle is very rapid, since the principal limitation on the charging rate is the minimum resistance of the tube 3!. Thus the major portion of each cycle is occupied by the discharge through the tube 3, and the frequency depends substantially only upon the discharge rate.

The discharge time is a function of the current through the tube 3 and resistor 5, the capacitance C of the capacitor l, and the difference between the maximum and minimum values of the voltage e2 across the capacitor l. The upper and lower limits of e2 must be maintained constant if the frequency is to be controlled solely by the adjustment of the voltage 431. It is evident from the foregoing description that the lower limit of 62 is set definitely by the bias 62.. Or-

dinary variations in the characteristics of the tubes 21 and 25 will have substantially no effect on the cathode follower action.

The upper limit of e2 is set by the maximum value of e5, This depends substantially only on the power supply voltage (which can .be closely regulated), because the tube II is cut off when e2 is at its peak. Similarly, the tubes I and 3| are always operating either at cutoff or saturation, where the efiects of variations in character-,- istics are at a minimum.

I claim as my invention:

1. A self-oscillatory relaxation oscillator including a capacitor and means for discharging said capacitor at a predetermined rate; two electron discharge tubes, each including at least an anode, a cathode, and a control grid, and means coupling the anode of each of said tubes to the control grid of the other, whereby conduction through either of said tubes prevents conduction through the other; a direct current source connected through the anode to cathode path of the first of said tubes to said capacitor, whereby conduction through said first tube charges said capacitor; means responsive to the voltage across said capacitor to stop conduction through said first tube and initiate conduction through said second tube when said voltage reaches a predetermined upper level, and means responsive to the voltage across said capacitor to stop conduction through said second tube and initiate conduction through said first tube when said capacitor is discharged to a predetermined lower voltage level.

2. A self-oscillatory relaxation oscillator in oludinga capacitor that is to be discharged during discharging periods and charged during charging periods, means for continuously discharging said capacitor between charging periods, means for cyclically charging said capacitor including a direct current source and an electron discharge tube with its space path connected between said source and said capacitor, and means for controlling the flow of current through said tube, said means including a cathode follower circuit responsive to the voltage across said capacitor and comprising a second electron discharge tube with its anode connected directly to said direct current source and a load resistor connected between its cathode and said source, a third tube with its anode connected to that of said first tube and its cathode connected to the cathode of said second tube, means providing a predetermined potential at the control grid of said third tube whereby conduction through said third tube occurs only upon discharge of said capacitor below a predetermined voltage level, a fourth tube with its anode coupled to the control grid of said first tube and its control grid coupled to the anodes of said first and third tubes, whereby conduction through said third tube stops conduction through said fourth tube, initiating conduction through said first tube, and cessation of conduction through said first tube initiates conduction through said fourth tube to maintain said first tube non-conducting as long as the voltage across said capacitor is above said predetermined level.

3. A self-oscillatory relaxation oscillator including a capacitor and means for comparatively slowly discharging said capacitor, means comprising a charging circuit for charging said capacitor, means associated with said charging circuit for providing a predetermined lower limit voltage therefor, means responsive to the voltage across said capacitor to initiate comparatively rapid charging of said capacitor through said charging circuit when said capacitor voltage decreases to said predetermined lower limit voltage, means associated with said charging circuit for providing a predetermined upper limit voltage, and means responsive to the voltage across said capacitor to stop said charging when said capacitor voltage reaches said predetermined upper limit voltage and to initiate said discharging of said capacitor.

4. A self-oscillatory relaxation oscillator including a capacitor and means for comparatively slowly discharging said capacitor, said means including an electron discharge tube with its space path connected across said capacitor and means controlling the internal resistance of said tube to maintain substantially constant current therethrough; means responsive to the voltage across said capacitor to initiate comparatively rapid charging of said capacitor when said voltage decreases to a predetermined minimum, and means responsive to the voltage across said capacitor to stop said charging when said voltage reaches a predetermined maximum and to initiate said discharging of said capacitor.

5. A self-oscillatory relaxation oscillator including a capacitor and means for discharging said capacitor, means comprising a charging circuit for charging said capacitor, means associated with said charging circuit for providing a predetermined lower limit voltage therefor, means responsive to the voltage across said capacitor to initiate charging of said capacitor through said charging circuit when said capacitor voltage decreases to said predetermined lower limit voltage,,

means associated with said charging circuit for,

providing a predetermined upper limit voltage,"

and means responsive to the voltage across said capacitor to stop said charging when said capacitor voltage reaches said predetermined uplfi'r limit voltage and to initiate said discharging of said capacitor.

6. A self-oscillatory relaxation oscillator including a capacitor and means for discharging.

said capacitor, means providing a predetermined lower limit bias voltage, means responsive totlf voltage across said capacitor to initiate charging Y of said capacitor when said capacitor voltage decreases to said predetermined lower limitbias voltage, said last-mentioned means including afirst electron discharge tube having an anode and having input electrodes comprising a cathode and a control grid, and further including means I applying said lower limit bias voltage to one input electrode of said first tube and means effectively} applying to the other input electrode of saidfirs t tube the voltage appearing on the terminal of said capacitor at which terminal the voltage-1" changes with respect to said lower limit bias" voltage during discharging of the capacitor, means providing a predetermined upper limit voltage means responsive to the voltage across said capacitor to stop said charging when said capacitor voltage reaches said predetermined upper limit voltage and to initiate said discharg- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name 3 Date 2,155,210 Young April 18, 1939 2,248,975 Faudell July 15, 1941 2,265,290 Knick Dec. 9, 1941 2,414,486 Rieke Jan. 21, 1947 2,448,069 Ames, Jr. et al. Aug. 31, 1948 2,448,070 Sunstein Aug. 31, 1948 

